1. Field of the Invention
This invention relates to polymers used in making paper suitable for use in computer printers, using water or solvent based inks, and, more particularly, to coated papers which exhibit advantageous water-fastness, excellent image quality, and dry fast.
2. Description of the Prior Art
The advent of color inkjet printing has been instrumental in fueling the print-on-demand revolution and has also created a number of challenges. Often, the surface of the desired media does not possess the necessary properties for accepting the ink-jet ink. This results in long dry times and/or a poor ink-jet image. It has long been recognized that a surface treatment or media coating plays a critical role in the final print quality. Numerous media coatings are known in the art. They may contain any number of components and often consist of more than one layer. These ink-receptive coatings generally contain at least one hydrophilic polymer; often poly(vinylpyrrolidone) (PVP). PVP brings many benefits to properly formulated media coatings including rapid ink dry time, excellent print quality, highly resolved circular dots, and high, uniform optical density. Furthermore, copolymers of vinylpyrrolidone (VP) along with other suitable comonomers, such as dimethylaminoethyl methacrylamide (DMAPMA), acrylic acid, or vinyl acetate, have been used separately or in conjunction with PVP, to further optimize performance. However, it is desired also to provide fast dry times of solvents in the inks, and water-fastness qualities for such films.
Landy, F., in U.S. Pat. No. 5,527,853, discloses a copolymer of dimethylaminopropyl methacrylamide (DMAPMA) and hydroxyethyl methacrylate (HEMA); however, without description of its preparation or further characterization.
What is described herein is an uncrosslinked or crosslinked copolymer of dimethylaminopropyl methacrylamide (DMAPMA) monomer and hydroxyethyl methacrylate (HEMA) monomer having residual monomer levels of  less than 2000 ppm and  less than 500 ppm, respectively, preferably in a weight ratio of DMAPMA:HEMA of about 60-70:40-30, prepared by copolymerizing the monomers in an aqueous-alcohol solvent, e.g. with isopropanol, wherein the desired weight ratio of DMAPMA:HEMA predetermines the weight ratio of water:alcohol to be used during the polymerization. In particular, a higher HEMA content in the copolymer requires a greater amount of alcohol in the cosolvent.
The product of the polymerization is an aqueous-alcoholic solution which has a pH of about 9-11, a solids content of about 29-31%, and a Brookfield viscosity of about 1,000 to 20,000 cps [LV #4, 12 rpm @25xc2x0 C.].
A substrate is coated with the copolymer of the invention, e.g. paper, glass or plastic, which is capable of absorbing the solvents, e.g. water or organic solvents, of digital printing inks rapidly, i.e. with dry times of  less than 1 min.